1. Field of the Invention
The present invention relates to a fuel cell stack comprising a stack body formed by stacking a plurality of power generation cells in a stacking direction. Each of the power generation cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. Each of the separators has a fluid flow field for allowing at least one of a reactant gas and a coolant to flow in a direction along a power generation surface. A fluid passage connected to the fluid flow field extends through the separators in the stacking direction. Terminal plates, insulating plates, and end plates are provided at opposite ends of the stack body.
2. Description of the Related Art
In general, a polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Normally, a predetermined numbers of membrane electrode assemblies and separators are stacked together alternately to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In some of power generation cells of the fuel cell stack, in comparison with the other power generation cells, the temperature is decreased easily due to heat radiation to the outside. For example, in the power generation cells provided at ends of the fuel cell stack in the stacking direction (hereinafter also referred to as the “end power generation cells”), since the heat is radiated to the outside from the terminal plates (current collecting plates) for collecting electrical charges generated in each of the power generation cells as electricity, or from the end plates for tightening the stacked power generation cells, the decrease in the temperature is significant.
Therefore, due to the decrease in the temperature, in the end power generation cells, in comparison with power generation cells in the central position of the fuel cell stack, water condensation occurs easily, and the water produced in the power generation cannot be discharged smoothly. Consequently, the power generation performance of the end power generation cells is low.
In an attempt to address the problem, for example, Japanese Laid-Open Patent Publication No. 8-203553 discloses a polymer electrolyte fuel cell as shown in FIG. 9. In the polymer electrolyte fuel cell, each of two tightening plates (end plates) 1 sandwiching a plurality of unit cells (not shown) includes a honeycomb plate 2. Packing plates 3a, 3b are provided on both surfaces of the honeycomb plate 2. Further, end plates 4a, 4b are stacked on the packing plates 3a, 3b. The honeycomb plate 2 includes a frame 2a and a honeycomb body 2b welded to the frame 2a. 
The hollow space in the honeycomb body 2b reduces the weight of the tightening plate 1, and improves the mechanical strength of the tightening plate 1 advantageously. Further, gases can flow through the hollow space in the honeycomb body 2b. According to the disclosure of Japanese Laid-Open Patent Publication No. 8-203553, with the use of the honeycomb body 2b, it is possible to achieve low thermal conductivity and thermal insulation by air. Thus, the tightening plate 1 does not radiate heat significantly.
However, in the conventional technique, since the tightening plate 1 is formed by stacking the honeycomb plate 2, the packing plates 3a, 3b, and the end plates 4a, 4b. Therefore, the number of components of the tightening plate 1 is large. The total number of components of the fuel cell stack is increased significantly, and the fuel cell stack cannot be assembled easily. Thus, the fuel cell stack cannot be produced economically.